Water levels in dried-out Lake Okeechobee matched a historic low Wednesday while firefighters battled a blaze burning on part of the exposed lake bottom.

The South Florida Water Management District expected the record of 8.97 feet from May 24, 2001, to evaporate by Thursday as the region's worst known drought continues. The average water level should be around 13 feet this time of year in the second-largest freshwater lake in the contiguous United States.

The 12,000-acre fire started Monday in the vegetation left to dry in the sun as waters receded from the lake's northwest rim, said Melissa Yunas, a spokeswoman for the Florida Division of Forestry.

"All the water is not there. Now it's just vegetation, all dried out, just sitting on the side of the lake," Yunas said.

The cause of the fire was unknown. It was about 50 percent contained, Yunas said.

Lake Okeechobee is a backup drinking water source for millions of people in South Florida and the lifeblood of the Everglades. The region is largely dependent on the lake during dry periods, when it can be used as a reservoir.

"The Everglades are also in a drought. We've totally lost the backup for the water supply for the east coast" of Florida, said Carol Wehle, executive director of the water management district.

An above-average rainy season is needed to replenish the lake and South Florida's groundwater system, officials said. Only about 7-8 inches of rain have fallen on the region in the past five months, about half of normal totals.

No significant rainfall was expected over the lake or the Kissimmee River valley to its north before Thursday night, according to the National Weather Service.

The drought has forced stringent water restrictions on homes and businesses in 13 counties, and four coastal wells were closed to prevent saltwater contamination.

The drought has allowed officials to begin clearing 500,000 cubic yards of rotted plant life and sediment from the southwest portion of the 730-square-mile lake to return its bottom to a more sandy base, improve water quality and restore wildlife habitat.